Thanks for repñying, steve!
What's a Plex server? As I see it, my tablet takes the video from the internat, then it sends it via wifi to either the modem through a different channel and the modem through another different channel toward the Chromecast or my tablet sends the file to the Chromecast directly via wifi with a different channel after passing the condition that there's an active internet connection. In any case, it's clear that the Chromecast is capable of bidirectional communication with my tablet (think of the configuration step).
My most refined question is: what's the exact route that video that comes from, say Youtube, takes until it gets to the tv? From there it might be easier to understand then what's happening underneath...